| Summary: | We report results of multi-transition observations and modeling of the hydrocarbon ring molecule cyclopropenylidene (C$\sb3$H$\sb2$). From a survey of the 1$\sb{10}$-1$\sb{01}$ (18 GHz) and 2$\sb{12}$-1$\sb{01}$ (85 GHz) transitions in the Galaxy, we have found C$\sb3$H$\sb2$ present in a variety of sources including cold, dark clouds, giant molecular clouds, the envelope of a carbon star, and diffuse clouds. Up to 10 transitions of C$\sb3$H$\sb2$ ranging in wavelength from 1.3 cm to 1.3 mm were observed in the dark clouds L1498, L134N, B335 and toward several positions in TMC-1. The Large Velocity Gradient (LVG) approximation was used to model the observations. Optical depth values of C$\sb3$H$\sb2$, estimated from C$\sp{13}$C$\sb2$H$\sb2$ observations, are necessary to constrain the results since the range in excitation energies of the observed C$\sb3$H$\sb2$ transitions does not contrast sufficiently. The molecular hydrogen density in TMC-1 is estimated to be 3.7 $\times$ 10$\sp4$ cm$\sp{-3}$, while the fractional abundance of C${\sb3}$H$\sb2$ relative to H$\sb2$ is 5.7 $\times$ 10$\sp{-9}$. Previous estimates assuming LTE conditions overestimate the abundance of C$\sb3$H$\sb2$. The abundance in the ridge component in Orion is estimated to be approximately 8 $\times$ 10$\sp{-10}$ cm $\sp{-2}$. Gas phase chemical models can reproduce the high C$\sb3$H$\sb2$ abundance found in dark clouds under assumptions such as steady state conditions with (C) / (O) / $>$ 1.0, conditions of earlier evolutionary time, or 'optimistic' rate coefficients. However, large deuteration ratios (0.05 to 0.15) create difficulties for gas phase models.
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